1. Field of the Invention
This invention relates to an electroconductive cross-linked polyolefin foam and to a method for the manufacture thereof.
2. Description of the Prior Art
Heretofore in the manufacture of resin foam, it has been customary to incorporate as an inorganic filler an oxide, a hydroxide, a carbonate, a sulfate, or a silicate of varying metal, a silicate mineral, or a silica mineral into the resin being foamed. The resin foam which is consequently produced varies with the kind of resin and that of the filler and possesses merits and demerits of its own. It is put to special applications which such characteristics particularly suit.
In recent years, the advances in integrated circuits (IC's) have been encouraging conversion of electric products into electronic countertypes. Despite their numerous features, the IC's suffer from the disadvantage that they are highly vulnerable to static electricity. The accidents experienced during the manufacture or assemblage of IC's are caused more often than not by the electrification of workers and the accidents during the packaging or carriage of finished IC's products are predominantly caused by the electrification of trays and containers.
For the purpose of protecting IC's against the static electricity due to contact of IC's with workers' bodies and the static electricity due to exposure of IC's to vibrations or frictions in the course of carriage, composite electroconductive materials based on various plastics have been utilized in IC cases, IC carrier magazines, etc.
In the circumstances, polyurethane foam has won recognition as a plastic foam possessing electroconductivity. As a technique available for the manufacture of polyurethane foam, there has been proposed a method which comprises foaming polyurethane, impregnating the resultant foam with a carbon paint, and drying the wet foam or a method which comprises foaming polyurethane and impregnating the resultant foam with a solution consisting of a water-dispersible carbon black-containing high molecular resin composition and a water-dispersible binder thereby affording an electroconductive polyurethane foam (Japanese Patent Publication SHO 52(1977)-36902).
The electroconductive polyurethane foam which is obtained by such a method as described above, however, suffers from exudation of carbon or loss of color. Thus, it calls for highly delicate handling and has the possibility of entailing the issue of environmental pollution. Further, the polyurethane foam is quite deficient in weatherability because it is produced from polyurethane. By this and other various reasons, the conventional polyurethane foam has been used under highly restricted conditions.
Polyolefin resins exhibit highly desirable weatherability as compared with polyurethane. They also enjoy outstanding stability to resist the actions of chemicals and water. Only if there is developed a method which is capable of conferring electroconductivety upon the cross-linked polyolefin foam enjoying such outstanding properties as mentioned above, the method is expected to materialize the manufacture of an electroconductive cross-linked polyolefin foam which will suffer from none of the aforementioned drawbacks inherent in the conventional electroconductive polyurethane foam, find utility in a notably widened range of applications, and prove to be highly useful.
According to literature, however, it has been held resignedly that the incorporation of carbon particles into the resin composition fails to confer the desired electroconductivity upon the finally produced foam because, during the blending of the resin composition with the carbon particles, the kneading machine such as a mixing roll exerts shearing stress upon the resin composition and, consequently, brings about destruction on the structure of carbon particles and on the chains of carbon particles. Thus, a review of patent literature reveals that none of the patent applications so far filed has covered an electroconductive cross-linked polyolefin foam of the type having carbon particles incorporated therein by kneading.